Method and system for selecting optimal satellites in view

ABSTRACT

A system and method for determining one or more satellites in view of a wireless device. A request for satellite assistance data may be received from a requesting entity and a reference location determined as a function of the request. A set of satellites may be determined as a function of the reference location. Subsequent cached information may be determined for each satellite in the set of satellites if cached information already existing for each satellite in the set of satellites has been cached for greater than a predetermined time period. This subsequent cached information may then replace the previously existing cached information for the reference location. One or more satellites in view of the wireless device may then be determined as a function of either the subsequent cached information or previously existing cached information, and assistance data may be provided to the requesting entity to determine an estimated location of the wireless device.

BACKGROUND

Radio communication systems generally provide two-way voice and datacommunication between remote locations. Examples of such systems arecellular and personal communication system (“PCS”) radio systems,trunked radio systems, dispatch radio networks, and global mobilepersonal communication systems (“GMPCS”) such as satellite-basedsystems. Communication in these systems is conducted according to apre-defined standard. Mobile devices or stations, also known ashandsets, portables or radiotelephones, conform to the system standardto communicate with one or more fixed base stations. It is important todetermine the location of such a device capable of radio communicationespecially in an emergency situation. In addition, in 2001 the UnitedStates Federal Communications Commission (“FCC”) required that cellularhandsets must be geographically locatable. This capability is desirablefor emergency systems such as Enhanced 911 (“E-911”). The FCC requiresstringent accuracy and availability performance objectives and demandsthat cellular handsets be locatable within 100 meters 67% of the timefor network based solutions and within 50 meters 67% of the time forhandset based solutions.

Current generations of radio communication generally possess limitedmobile device location determination capability. In one technique, theposition of the mobile device is determined by monitoring mobile devicetransmissions at several base stations. From time of arrival orcomparable measurements, the mobile device's position may be calculated.However, the precision of this technique may be limited and, at times,may be insufficient to meet FCC requirements. In another technique, amobile device may be equipped with a receiver suitable for use with aGlobal Navigation Satellite System (“GNSS”) such as the GlobalPositioning System (“GPS”). GPS is a radio positioning system providingsubscribers with highly accurate position, velocity, and time (“PVT”)information.

FIG. 1 is a schematic representation of a constellation 100 of GPSsatellites 101. With reference to FIG. 1, GPS may include aconstellation of GPS satellites 101 in non-geosynchronous orbits aroundthe earth. The GPS satellites 101 travel in six orbital planes 102 withfour of the GPS satellites 101 in each plane. Of course, a multitude ofon-orbit spare satellites may also exist. Each orbital plane has aninclination of 55 degrees relative to the equator. In addition, eachorbital plane has an altitude of approximately 20,200 km (10,900 miles).The time required to travel the entire orbit is just under 12 hours.Thus, at any given location on the surface of the earth with clear viewof the sky, at least five GPS satellites are generally visible at anygiven time.

With GPS, signals from the satellites arrive at a GPS receiver and areconventionally utilized to determine the position of the receiver. GPSposition determination is made based on the time of arrival (“TOA”) ofvarious satellite signals. Each of the orbiting GPS satellites 101broadcasts spread spectrum microwave signals encoded with satelliteephemeris information and other information that allows a position to becalculated by the receiver. Presently, two types of GPS measurementscorresponding to each correlator channel with a locked GPS satellitesignal are available for GPS receivers. The two carrier signals, L1 andL2, possess frequencies of 1.5754 GHz and 1.2276 GHz, or wavelengths of0.1903 m and 0.2442 m, respectively. The L1 frequency carries thenavigation data as well as the standard positioning code, while the L2frequency carries the P code and is used for precision positioning codefor military applications. The signals are modulated using bi-phaseshift keying techniques. The signals are broadcast at precisely knowntimes and at precisely known intervals and each signal is encoded withits precise transmission time. There is also an L2C signal beingtransmitted by several satellites. The L2C signal is a second civilianfrequency transmitted by GPS satellites. L1 transmits the CoarseAcquisition (“C/A”) code. L2C transmits L2CM (civil-moderate) and L2CL(civil long) codes. These codes allow a device to differentiate betweensatellites that are all transmitting on the same frequency. The C/A codeis 1 milliseconds long, the L2CM is 20 milliseconds long and the L2CL is1.5 seconds long. The L2C codes provide a more robust cross-correlationperformance so that reception of weak GPS signals is less affected bysimultaneously received strong GPS signals. The civil navigation message(“CNAV”) is the broadcast model that can be transmitted on the L2C andprovides a more accurate and frequent message than the legacy navigationmessage (“NAV”).

GPS receivers measure and analyze signals from the satellites, andestimate the corresponding coordinates of the receiver position, as wellas the instantaneous receiver clock bias. GPS receivers may also measurethe velocity of the receiver. The quality of these estimates dependsupon the number and the geometry of satellites in view, measurementerror and residual biases. Residual biases generally include satelliteephemeris bias, satellite and receiver clock errors, and ionospheric andtropospheric delays. If receiver clocks were perfectly synchronized withthe satellite clocks, only three range measurements would be needed toallow a user to compute a three-dimensional position. This process isknown as multilateration. However, given the engineering difficultiesand the expense of providing a receiver clock whose time is exactlysynchronized, conventional systems generally account for the amount bywhich the receiver clock time differs from the satellite clock time whencomputing a receiver's position. This clock bias is determined bycomputing a measurement from a fourth satellite using a processor in thereceiver that correlates the ranges measured from each satellite. Thisprocess requires four or more satellites from which four or moremeasurements can be obtained to estimate four unknowns x, y, z, b. Theunknowns are latitude, longitude, altitude and receiver clock offset.The amount b, by which the processor has added or subtracted time, isthe instantaneous bias between the receiver clock and the satelliteclock. It is possible to calculate a location with only three satelliteswhen additional information is available. For example, if the altitudeof the handset or mobile device is well known, then an arbitrarysatellite measurement may be included that is centered at the center ofthe earth and possesses a range defined as the distance from the centerof the earth to the known altitude of the handset or mobile device. Thealtitude of the handset may be known from another sensor or frominformation from the cell location in the case where the handset is in acellular network.

Traditionally, satellite coordinates and velocity have been computedinside the GPS receiver. The receiver obtains satellite ephemeris andclock correction data by demodulating the satellite broadcast messagestream. The satellite transmission contains more than 400 bits of datatransmitted at 50 bits per second. The constants contained in theephemeris data coincide with Kepler orbit constants requiring manymathematical operations to turn the data into position and velocity datafor each satellite. In one implementation, this conversion requires 90multiplies, 58 adds and 21 transcendental function cells (sin, cos, tan)in order to translate the ephemeris into a satellite position andvelocity vector at a single point, for one satellite. Most of thecomputations require double precision, floating point processing.

Thus, the computational load for performing the traditional calculationis significant. The mobile device must include a high-level processorcapable of the necessary calculations, and such processors arerelatively expensive and consume large amounts of power. Portabledevices for consumer use, e.g., a cellular phone or comparable device,are preferably inexpensive and operate at very low power. These designgoals are inconsistent with the high computational load required for GPSprocessing. Further, the slow data rate from the GPS satellites is alimitation. GPS acquisition at a GPS receiver may take many seconds orseveral minutes, during which time the receiver circuit and processor ofthe mobile device must be continuously energized. Preferably, tomaintain battery life in portable receivers and transceivers such asmobile cellular handsets, circuits are de-energized as much as possible.The long GPS acquisition time can rapidly deplete the battery of amobile device. In any situation and particularly in emergencysituations, the long GPS acquisition time is inconvenient.

Assisted-GPS (“A-GPS”) has gained significant popularity recently inlight of stringent time to first fix (“TTFF”), i.e., first positiondetermination and sensitivity, requirements of the FCC E-911regulations. In A-GPS, a communications network and associatedinfrastructure may be utilized to assist the mobile GPS receiver, eitheras a standalone device or integrated with a mobile station or device.The general concept of A-GPS is to establish a GPS reference network(and/or a wide-area D-GPS network or a wide area reference network(“WARN”)) including receivers with clear views of the sky that mayoperate continuously. This reference network may also be connected withthe cellular infrastructure, may continuously monitor the real-timeconstellation status, and may provide data for each satellite at aparticular epoch time. For example, the reference network may provideephemeris information, UTC model information, ionosphere modelinformation, and other broadcast information to the cellularinfrastructure. As one skilled in the art would recognize, the GPSreference receiver and its server (or position determining entity) maybe located at any surveyed location with an open view of the sky.Typical A-GPS information may include data for determining a GPSreceiver's approximate position, time synchronization mark, satelliteephemerides, various model information and satellite dopplers. DifferentA-GPS services may omit some of these parameters; however, anothercomponent of the supplied information is the identification of thesatellites for which a device or GPS receiver should search. From suchassistance data, a mobile device will attempt to search for and acquiresatellite signals for the satellites included in the assistance data. Ithowever, satellites are included in the assistance data that are notmeasurable by the mobile device (e.g., the satellite is no longervisible, etc.), then the mobile device will waste time and considerablepower attempting to acquire measurements for the satellite.

A-GPS handset implementations generally rely upon provided assistancedata to indicate which satellites are visible or “in view.” As afunction of the assistance data, a mobile device will attempt to searchfor and acquire satellite signals for the satellites included in theassistance data. A-GPS positioning may also rely upon the availabilityof a coarse location estimate to seed the positioning method. Thiscoarse location estimate or reference location may be utilized todetermine a likely set of satellites from which a respective mobiledevice may receive signals. For example, upon receipt of a locationrequest, satellites in view of the reference location for a wirelessdevice may be determined so that a location determining entity, e.g.,location information server (“LIS”) or mobile location center (“MLC”)can provide assistance data for those respective satellites. Once thesatellites in view are determined, assistance data for those satellitesmay be collated (or calculated), encoded into the relevant protocol, andprovided to the handset.

There is, however, a need for a method and system for optimizing theprocess of calculating the satellites in view of the device to achieve amore efficient performance that would overcome the deficiencies of theprior art. Additionally, the performance of the acquisition assistancedetermination may be increased by optimizing the calculation ofsatellite velocities. Therefore, an embodiment of the present subjectmatter provides a method for determining one or more satellites in viewof a wireless device. The method may comprise receiving a request forsatellite assistance data from a requesting entity and determining areference location as a function of the request. A set of satellites maybe determined as a function of the reference location. Subsequent cachedinformation may be determined for each satellite in the set ofsatellites if initial cached information for each satellite has beencached for greater than a predetermined time period. One or moresatellites in view of the wireless device may then be determined as afunction of either the subsequent cached information or initial cachedinformation.

Another embodiment of the present subject matter may provide a methodfor determining an estimated location of a wireless device. The methodmay include receiving a request for satellite assistance data from awireless device and determining a reference location of the wirelessdevice. A set of satellites may be determined as a function of thereference location, and subsequent cached information determined foreach satellite in the set of satellites if initial cached informationfor each satellite in the set of satellites has been cached for greaterthan a predetermined time period. One or more satellites in view of thewireless device may be determined as a function of either the subsequentcached information or initial cached information. Assistance data maythen be transmitted to the wireless device, the assistance dataincluding information from the one or more satellites in view, and anestimated location of the wireless device determined from the includedinformation.

A further embodiment of the present subject matter provides a method fordetermining one or more satellites in view of a wireless device. Themethod may include receiving a request for satellite assistance datafrom a requesting entity, and determining a reference location as afunction of the request. A first set of satellites in view may bedetermined as a function of the reference location if there isinformation available from a first cached set of satellites, the firstcached set of satellites being cached for less than a predetermined timeperiod. A second set of satellites in view may be determined as afunction of the reference location if the first cached set of satelliteshas been cached for greater than the predetermined time period anddetermining additional cached information for each satellite in thesecond set of satellites as a function of the reference location. Inanother embodiment, information of the second set of satellites in viewmay be cached for the reference location.

An additional embodiment of the present subject matter provides a methodfor determining an estimated location of a wireless device. The methodmay comprise receiving a request for satellite assistance data from awireless device and determining a reference location of the wirelessdevice. A first set of satellites in view may be determined as afunction of the reference location if there is information availablefrom a first cached set of satellites, the first cached set ofsatellites being cached for less than a predetermined time period. Asecond set of satellites in view may be determined as a function of thereference location if the first cached set of satellites has been cachedfor greater than the predetermined time period and determiningadditional cached information for each satellite in the second set ofsatellites as a function of the reference location. Assistance data maythen be transmitted to the wireless device, the assistance dataincluding information from the first or second sets of satellites inview, and an estimated location determined for the wireless device fromthe included information.

One embodiment of the present subject matter may provide a system fordetermining one or more satellites in view of a wireless device. Thesystem may comprise circuitry for receiving a request for satelliteassistance data from a requesting entity and circuitry for determining areference location as a function of the request. The system may furtherinclude circuitry for determining a set of satellites as a function ofthe reference location and circuitry for caching initial information foreach satellite in the set of satellites. The system may also comprisecircuitry for determining and caching subsequent cached information foreach satellite in the set of satellites if the cached initialinformation has been cached for greater than a predetermined timeperiod, as well as circuitry for determining one or more satellites inview of the wireless device as a function of either the subsequentcached information or initial cached information.

These embodiments and many other objects and advantages thereof will bereadily apparent to one skilled in the art to which the inventionpertains from a perusal of the claims, the appended drawings, and thefollowing detailed description of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a constellation of GPSsatellites.

FIG. 2 is an algorithm according to one embodiment of the presentsubject matter.

FIG. 3 is an algorithm according to another embodiment of the presentsubject matter.

FIG. 4 is an algorithm according to a further embodiment of the presentsubject matter.

FIG. 5 is an algorithm according to an embodiment of the present subjectmatter.

FIG. 6 is a schematic representation for implementing one embodiment ofthe present subject matter.

DETAILED DESCRIPTION

With reference to the figures where like elements have been given likenumerical designations to facilitate an understanding of the presentsubject matter, the various embodiments of a method and system forselecting optimal satellites in view for the location of a wirelessdevice are herein described.

The disclosure relates to a mobile appliance or device and a locationdetermining system using satellite signals and/or measurements of thesesatellite signals. Exemplary devices may include, but are not limitedto, a cellular device, text messaging device, computer, portablecomputer, vehicle locating device, vehicle security device,communication device, and wireless transceiver. The satellites may beconsidered as part of a Global Navigation Satellite System (“GNSS”),such as, but not limited to, the U.S. Global Positioning System (“GPS”).While the following description references the GPS system, this in noway should be interpreted as limiting the scope of the claims appendedherewith. As is known to those of skill in the art, other GNSS systemsoperate, for the purposes of this disclosure, similarly to GPS, such as,but not limited to, the European Satellite project, Galileo; the Russiansatellite navigation system, GLONASS; the Japanese Quasi-ZenithSatellite System (“QZSS”), and the Chinese satellite navigation andpositioning system called Beidou (or Compass). Therefore, references inthe disclosure to GPS and/or GNSS, where applicable, as known to thoseof skill in the art, apply to the above-listed GNSS systems as well asother GNSS systems not listed above.

Generally wireless A-GPS devices or handsets have a low time to firstfix (“TTFF”) as the devices are supplied with assistance data from anexemplary communications network to assist in locking onto or acquiringsatellites quickly. Exemplary A-GPS devices may include, but are notlimited to, a cellular device, text messaging device, computer, portablecomputer, vehicle locating device, vehicle security device,communication device, and wireless transceiver. These devices mayprovide satellite measurements back to a location determining system toperform a position calculation. Exemplary network elements that supplythe assistance data and/or perform the position calculation may be alocation determining system such as a Mobile Location Center (“MLC”),location information server or system (“LIS”), or other comparablenetwork element. The location determining system may generally be a nodein a wireless network that performs the location of a mobile device.

Typical A-GPS information includes data for determining a GPS receiver'sapproximate position, time synchronization mark, satellite ephemerides,and satellite dopplers. Different A-GPS services may omit some of theseparameters; however, another component of the supplied information maybe the identification of the satellites for which a device or GPSreceiver should search. Acquisition assistance data for A-GPS isgenerally comprised of several fields (GPS time of week, satellite ID,Doppler, Code Phase, satellite elevation, azimuth, etc.) having aconsistent content (but not encoding format) across the variousPositioning Calculation Application Part (“PCAP”), Radio Resource LCSProtocol (“RRLP”) and Position Determination for Dual Mode (“PDDM”)specifications. Generally, a message sequence starts with a request forGPS assistance data arriving in an MLC with an estimated handsetlocation. This may include the horizontal uncertainty, the altitude andthe altitude uncertainty. In some instances, the estimated handsetlocation may be the location of a serving cell tower. The MLC may thencalculate the assistance data and return it to the handset. Thus, oncethe satellites in view are determined, assistance data for thesatellites may be collated, encoded into the relevant protocol, andprovided to the respective wireless device. The handset may then use theassistance data to lock onto the respective satellite(s) and report themeasurements to the server. Of course, embodiments of the presentsubject matter may determine the reference location of the wirelessdevice in any number of ways and such a disclosure should not limit thescope of the claims appended herewith. For example, the referencelocation may be determined by any of the methods described in co-pendingU.S. application Ser. No. ______, filed ______, entitled “System andMethod for Determining a Reference Location of a Mobile Device,” theentirety of which is incorporated herein by reference. Further, thereference location may be determined as a function of signals receivedfrom a terrestrial radio frequency (“RF”) technology such as, but notlimited to, a cellular technology or a different type of network such asWiFi or WiMax.

Using the determined reference location, the MLC may then determine aset of satellites in view at a particular time. This may be accomplishedby determining the location and/or velocity of each of the satellites inview at that particular time, determining the elevation for eachsatellite from the reference location, selecting a set of satellites inview as those satellites having an elevation greater than apredetermined mask angle, and/or piecing together acquisition assistancedata if necessary. An exemplary method of determining the location foreach satellite is described in the GPS Interface Control Document(“ICD”), the entirety of which is incorporated herein by reference.Generally, this procedure utilizes a satellite's ephemeris parametersand places these parameters into a series of equations. A portion ofthis procedure to determine a satellite's location solves Kepler'snon-linear equation. The general method of solving Kepler's equationutilizes an iterative solution; however, the comparatively large numberof steps results in a relatively expensive and time-consuming operation.

The coordinate reference frame employed by GPS is the World GeodeticSystem 84 (“WGS 84”) reference frame. Certain embodiments of the presentsubject matter may determine the location of satellites in WGS 84Earth-Centered Earth-Fixed (“ECEF”) coordinates. The reference locationmay be in the same coordinate reference frame, and the elevation foreach satellite may be determined therefrom. An exemplary mask angle maybe configured as an elevation in degrees greater than which it is likelythe respective mobile device can acquire and lock onto a respectivesatellite(s). For example, the mask angle may be set to zero but mayalso be set to a number above the horizon such as five, ten, fifteen ormore degrees.

When assistance data is required for the determination of the locationof a wireless device, a server should determine which satellites are inview to provide appropriate assistance data as described above. For ahigh throughput system such as an MLC (e g., 200 TPS) these calculationsmay be performed regularly. As discussed above, one of the data typesfor handset-assisted GPS is acquisition assistance. To determine thepredicted Doppler of a satellite from a certain reference location, thevelocity of the satellite may be determined as a three dimensionalvector. Doppler may be estimated from the difference of a user motionand the motion of the satellite. To generate acquisition assistance itmay be assumed that a handset is stationary on the surface of therotating earth where the Doppler, in hertz, is calculated as the dotproduct of the difference in velocities and the estimated line of sightvector:

$\begin{matrix}{D = {{- \left( {\frac{v_{s} - v_{r}}{c} \cdot {\hat{l}}_{s}} \right)}f}} & (1)\end{matrix}$

where v_(s) represents the velocity of the satellite, v_(r) representsthe velocity of the receiver, c is the speed of light, l_(s) representsthe estimated line of sight unit vector, and f represents thetransmitted frequency of L1 which is 1575.42 MHz. The c and f scalarsmay simply convert the result to Hz. The line of sight vector may becalculated as the difference between the satellite and the receiverlocation divided by the norm of the difference:

$\begin{matrix}{{\hat{l}}_{s} = \frac{r_{s} - {\hat{r}}_{r}}{{r_{s} - {\hat{r}}_{r}}}} & (2)\end{matrix}$

where r_(s) represents the location of the satellite in Earth CenteredEarth Fixed (“ECEF”) coordinates and r_(r) represents the location ofthe receiver. The frequency may be corrected to account for clock erroras described in Xu, Guichang, 2003, GPS Theory, Algorithms andApplications, Springer at p. 75, the entirety of which is incorporatedherein by reference.

The velocity of the receiver is zero since the equation is in ECEFcoordinates. The velocity of the satellite v_(s) may be determined inthe perifocal coordinate system (inertial) and then converted back tothe ECEF coordinate system (rotating). The polar equation of a conic maybe utilized to determine the semi-latus rectum or ρ:

ρ=r(1+ecos v)   (3)

where r represents the norm of the difference between the location ofthe satellite and the receiver, e represents the eccentricity providedstraight from the ephemeris, and v represents the true anomaly at epochwhich is also from the ephemeris.

As discussed in Bate, et al., 1971, Fundamentals of Astrodynamics, NewYork, Dover Publications, Inc., the entirety of which is incorporatedherein by reference, the following formulas may be derived providingvelocity with respect to the P and Q unit vectors of the perifocalcoordinate system:

$\begin{matrix}{v_{p} = {\sqrt{\frac{\mu}{\rho}}\left( {{- \sin}\; v} \right)}} & (4) \\{v_{q} = {\sqrt{\frac{\mu}{\rho}}\left( {e + {\sin \; v}} \right)}} & (5)\end{matrix}$

where μ represents WGS 84 universal gravitation parameter from the WGS84 specification. The velocity may then be converted from the perifocalcoordinate system to the geocentric-equatorial frame (v_(i), v_(j),v_(k)) using the rotation matrix R as shown:

$\begin{matrix}{\begin{bmatrix}v_{i} \\v_{j} \\v_{k}\end{bmatrix} = {\overset{\sim}{R}\begin{bmatrix}v_{p} \\v_{q} \\v_{w}\end{bmatrix}}} & (6)\end{matrix}$

v_(w) is zero as a result of being in the perifocal coordinate systemand the W axis being in the orbit plane (thus, the velocity of thesatellite is zero in this direction). The rotation matrix may berepresented as:

$\begin{matrix}{\overset{\sim}{R} = \begin{bmatrix}R_{11} & R_{12} & R_{13} \\R_{21} & R_{22} & R_{23} \\R_{31} & R_{32} & R_{33}\end{bmatrix}} & (7)\end{matrix}$

where the elements of the rotation matrix are as follows:

R₁₁=cos Ω cos ω−sin Ω sin ω cos i   (8)

R₁₂−cos Ω cos ω−sin Ω cos ω cos i   (9)

R₁₃=sin Ω sin i   (10)

R₂₁=sin Ω cos ω+cos Ω sin ω cos i   (11)

R₂₂=−sin Ω sin ω+cos Ω cos ω cos i   (12)

R₂₃=−cos Ω sin i   (13)

R₃₁=sin ω sin i   (14)

R₃₂=cos ω sin i   (15)

R₃₃=cos i   (16)

where ω represents the argument of perigee from the ephemeris, Ωrepresents the longitude of the ascending node which is calculated fromthe ephemeris, and i represents the corrected inclination which is alsocalculated from the ephemeris.

Expanding the matrix expression provides:

$\begin{matrix}{\begin{bmatrix}v_{i} \\v_{j} \\v_{k}\end{bmatrix} = {\begin{bmatrix}R_{11} & R_{12} & R_{13} \\R_{21} & R_{22} & R_{23} \\R_{31} & R_{32} & R_{33}\end{bmatrix}\begin{bmatrix}v_{p} \\v_{q} \\v_{w}\end{bmatrix}}} & (17)\end{matrix}$

Multiplying matrices provides the following expressions in thegeocentric equatorial frame for v_(i), v_(j), and v_(k):

v_(i)=R₁₁ v _(p)+R₁₂ v _(q)+0   (18)

v _(j)=R₂₁ v _(p)+R₂₂ v _(q)+0   (19)

v _(k)=R₃₁ v _(p)+R₃₂ v _(q)+0   (20)

Of course, the R terms may be expanded appropriately. Converting theexpanded velocities to the ECEF reference frame (v_(x), V_(y), v_(z))from the inertial reference frame provides the following matrixnotation:

$\begin{matrix}{\begin{bmatrix}v_{x} \\v_{y} \\v_{z}\end{bmatrix} = {\begin{bmatrix}v_{i} \\v_{j} \\v_{k}\end{bmatrix} - \begin{bmatrix}{{- \overset{.}{\Omega}}y} \\{\overset{.}{\Omega}x} \\0\end{bmatrix}}} & (21)\end{matrix}$

where the matrix on the right hand side of Equation (21) represents theresult of the cross product of the omega matrix (the Earth's rotationrate) and the r matrix (the coordinates of the user). The {dot over (Ω)}term represents the WGS 84 constant for the Earth's rotation rate inradians per second.

This velocity and/or location may be optimized by various embodiments ofthe present subject matter. For example, exemplary satellites generallyorbit at high speeds; however, the angular speed relative to a mobiledevice on the surface of the Earth is slow. Thus, for a one minute timedifference, the elevation of an exemplary satellite may only change byone or two degrees. In addition, the satellite velocity does not changesignificantly. Embodiments of the present subject matter may optimizethe determination of visible satellites by storing or caching thelocation of the satellites (e.g., in ECEF coordinates) and/or velocitiesof the satellites and recalculating their respective locations and/orvelocities as a function of a predetermined time period. An exemplarytime period may be, but is not limited to, five minutes. Suchembodiments would save a considerable amount of processing and expense.Therefore, when the satellites in view of a mobile device are required,the process described above may be utilized but the location and/orvelocity of any one or plural satellites may be used from the cache.Thus, the assistance data being provided and employed in such a systemwould be current, but the respective satellite locations and velocitieswould not be recalculated each time,

Another embodiment of the present subject matter may cache informationfrom satellites in view as a function of information provided by aterrestrial source. For example, when a location query or request isreceived from a mobile device, an embodiment may cache the satellitesassociated with the cell identification in the request. The cached listof satellites in view from that particular cell may then be utilized forsubsequent queries for that cell until a predetermined time periodpasses. If a query comes in for that cell after its respective cache hasexpired (e.g., five minutes), then the satellites in view may bedetermined either directly from the ephemeris information or optimallyusing the cached locations of the satellites.

FIG. 2 is an algorithm 200 according to one embodiment of the presentsubject matter. With reference to FIG. 2, at step 210, a request forsatellite assistance data may be received from a requesting entity, andat step 220 a reference location determined as a function of therequest. The requesting entity may be a wireless device or a locationbased service. Exemplary satellites may be part of a GNSS such as, butnot limited to, GPS, Galileo system, GLONASS system, QZSS, Beidousatellite system, Compass satellite system, and combinations thereof. Anexemplary device may be a cellular device, text messaging device,computer, portable computer, vehicle locating device, vehicle securitydevice, communication device, and wireless transceiver. At step 230, aset of satellites may be determined as a function of the referencelocation. In one embodiment, step 230 may include determining at leastone of a location and velocity for each satellite in the set ofsatellites, and determining an elevation for each satellite in the setof satellites as a function of the reference location, wherein eachsatellite in the set has an elevation greater than a predeterminedthreshold. An exemplary threshold may be, but is not limited to, a valuebetween zero and five degrees above the horizon of the Earth.

Subsequent cached information for each satellite in the set ofsatellites may then be determined if initial cached information for eachsatellite in the set of satellites has not been cached or has beencached for greater than a predetermined time period at step 240. Theinitial cached information and subsequent cached information may be, butis not limited to, satellite position information, ephemerisinformation, satellite velocity information, time information, clockcorrection information, satellite locations, and combinations thereof.The predetermined time period is configurable and may be approximatelyfive minutes, less than five minutes, or greater than five minutes.Information may also be cached for additional sets of satellites atintervals defined by the predetermined time period. The algorithm 200may also include caching the determined subsequent cached information inplace of the initial cached information. One or more satellites in viewof the wireless device may then be determined at step 250 as a functionof either the subsequent cached information or initial cachedinformation.

In another embodiment, assistance data may be transmitted to a wirelessdevice, the assistance data including information from the one or moresatellites in view at step 252, and an estimated location of thewireless device determined from the information at step 254. In afurther embodiment, an estimated location of a wireless device may bedetermined as a function of information from the determined one or moresatellites in view at step 256, and then the estimated locationtransmitted to the requesting entity at step 258.

FIG. 3 is another algorithm 300 according to an embodiment of thepresent subject matter. With reference to FIG. 3, at step 310, a requestfor satellite assistance data from a requesting entity may be received,and a reference location determined as a function of the request at step320. An exemplary requesting entity may be, but is not limited to, awireless device or a location based service. At step 330, a first set ofsatellites in view may be determined as a function of the referencelocation if there is information available from a first cached set ofsatellites, the first cached set of satellites being cached for lessthan a predetermined time period. At step 340, a second set ofsatellites in view may be determined as a function of the referencelocation if the first cached set of satellites has been cached forgreater than the predetermined time period and determining additionalcached information for each satellite in the second set of satellites asa function of the reference location. One or more satellites the firstand second sets of satellites may be different, each of the satellitesmay be the same between the sets, or the sets may be mutually exclusive.Exemplary information for cached satellites may be, but are not limitedto, satellite position information, ephemeris information, satellitevelocity information, time information, clock correction information,satellite locations, and combinations thereof. Of course, thepredetermined time period is configurable as discussed above. In afurther embodiment, the algorithm 300 may include caching information ofthe second set of satellites in view for the reference location.

In another embodiment, assistance data may be transmitted to a wirelessdevice, the assistance data including information from the first orsecond sets of satellites in view at step 342, and an estimated locationof the wireless device determined from the information at step 344. In afurther embodiment, an estimated location of a wireless device may bedetermined as a function of information from the first or second sets ofsatellites in view at step 346, and then the estimated locationtransmitted to the requesting entity at step 348. The algorithm 300 mayalso cache information for additional sets of satellites at intervalsdefined by the predetermined time period.

FIG. 4 is another algorithm 400 according to a further embodiment of thepresent subject matter. With reference to FIG. 4, at step 410, a requestfor satellite assistance data may be received from a wireless device,and at step 420 a reference location determined for the wireless device.At step 430, a set of satellites may be determined as a function of thereference location. Subsequent cached information for each satellite inthe set of satellites may then be determined if initial cachedinformation for each satellite in the set of satellites has been cachedfor greater than a predetermined time period at step 440. One or moresatellites in view of the wireless device may then be determined at step450 as a function of either the subsequent cached information or initialcached information. Assistance data may be transmitted to a wirelessdevice, the assistance data including information from the one or moresatellites in view at step 460, and an estimated location of thewireless device determined from the information, at step 470.

FIG. 5 is another algorithm 500 according to a further embodiment of thepresent subject matter. With reference to FIG. 5, at step 510, a requestfor satellite assistance data from a wireless device may be received,and a reference location for the wireless device determined at step 520.At step 530, a first set of satellites in view may be determined as afunction of the reference location if there is information availablefrom a first cached set of satellites, the first cached set ofsatellites being cached for less than a predetermined time period. Atstep 540, a second set of satellites in view may be determined as afunction of the reference location if the first cached set of satelliteshas been cached for greater than the predetermined time period anddetermining additional cached information for each satellite in thesecond set of satellites as a function of the reference location.Assistance data may be transmitted to the wireless device, theassistance data including information from the first or second sets ofsatellites in view at step 550, and an estimated location of thewireless device determined from the information at step 560.

FIG. 6 is a schematic representation for implementing one embodiment ofthe present subject matter. With reference to FIG. 6, a satellite system610 communicates with a ground system 620. The ground system 620 mayinclude a cellular network having a location center 621. The locationcenter 621 may be an MLC, LIS or a central office configured tocommunicate with a telecommunication network 622 and at least one basestation 623. In one embodiment of the present subject matter, a device624 communicates with the base station 623 to acquire GPS assistancedata. The location center 621 may or may not receive a preliminaryestimate of the receiver's location or boundary thereof on the basis ofthe receiver's cell site or other area, such as the boundary of thecommunications network or an area or region such as, but not limited to,city, municipality, county, state, country, or continent. The locationcenter 621 may also determine a plurality of satellites as a function ofthis boundary or region. This information may then be transmitted orrelayed to the mobile receiver and utilized for location determination.The location center 621 may also receive satellite information from GPSsatellites. The satellite information may include the satellite'sbroadcast ephemeris information of the broadcasting satellite or that ofall satellites or that of selected satellites. The location center 621may relay the information back to the device 624 or use the information,either singularly or along with some preliminary estimation of thedevice's location, to assist the device in a geographic locationdetermination. In another embodiment, any one or plural stepsillustrated in FIGS. 2, 3, 4 and 5 may be implemented at the locationcenter 621 and communicated to the device 624. Of course, the estimatedlocation of the device 624 may also be determined as a function ofadditional signals provided by the network 622. The location center 621or another remote device or server (not shown) may include circuitry foroptimizing the information for any number of satellites by cachinglocations of each satellite and determining further cached locations ofeach satellite. The system may include a transmitter for transmittingassistance data to the device 624, the assistance data includinglocation and/or velocity information from an optimized set of satellitesor for transmitting an estimated location of the device 624 to arequesting entity.

As shown by the various configurations and embodiments illustrated inFIGS. 1-6, a method and system for selecting optimal satellites forA-GPS location of a device are herein described.

While preferred embodiments of the present subject matter have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

1. A method for determining one or more satellites in view of a wirelessdevice comprising: (a) receiving a request for satellite assistance datafrom a requesting entity; (b) determining a reference location as afunction of the request; (c) determining a set of satellites as afunction of the reference location; (d) determining subsequent cachedinformation for each satellite in the set of satellites if initialcached information for each satellite in the set of satellites has beencached for greater than a predetermined time period; and (e) determiningone or more satellites in view of the wireless device as a function ofeither the subsequent cached information or initial cached information.2. The method of claim 1 wherein the initial cached information andsubsequent cached information is selected from the group consisting of:satellite position information, ephemeris information, satellitevelocity information, time information, clock correction information,satellite locations, and combinations thereof.
 3. The method of claim 1further comprising the steps of: (f) transmitting assistance data to awireless device, the assistance data including information from the oneor more satellites in view; and (g) determining an estimated location ofthe wireless device from the information.
 4. The method of claim 1further comprising the steps of: (f) determining an estimated locationof a wireless device as a function of information from the determinedone or more satellites in view; and (g) transmitting the estimatedlocation to the requesting entity.
 5. The method of claim 1 wherein therequesting entity is a wireless device or a location based service. 6.The method of claim 1 further comprising the step of caching thedetermined subsequent cached information in place of the initial cachedinformation.
 7. The method of claim 1 wherein the step of determining aset of satellites further comprises: (i) determining at least one of alocation and velocity for each satellite in the set of satellites; and(ii) determining an elevation for each satellite in the set ofsatellites as a function of the reference location, wherein eachsatellite in the set has an elevation greater than a predeterminedthreshold.
 8. The method of claim 7 wherein the predetermined thresholdis between zero and five degrees above the horizon of the Earth.
 9. Themethod of claim 1 wherein the predetermined time period is approximatelyfive minutes.
 10. The method of claim 1 wherein the predetermined timeperiod is greater than five minutes.
 11. The method of claim 1 furthercomprising the step of caching information for additional sets ofsatellites at intervals defined by the predetermined time period. 12.The method of claim 1 wherein the satellites are part of a GlobalNavigation Satellite System (“GNSS”) selected from the group consistingof: a Global Positioning System (“GPS”), Galileo system, GLONASS system,Quasi-Zenith Satellite System (“QZSS”), Beidou satellite system, Compasssatellite system, and combinations thereof.
 13. The method of claim 1wherein the device is selected from the group consisting of: cellulardevice, text messaging device, computer, portable computer, vehiclelocating device, vehicle security device, communication device, andwireless transceiver.
 14. A system for determining a set of satellitesin view of a wireless device comprising: (a) circuitry for receiving arequest for satellite assistance data from a requesting entity; (b)circuitry for determining a reference location as a function of therequest; (c) circuitry for determining a set of satellites as a functionof the reference location; (d) circuitry for caching initial informationfor each satellite in the set of satellites; (e) circuitry fordetermining and caching subsequent cached information for each satellitein the set of satellites if the cached initial information has beencached for greater than a predetermined time period; and (f) circuitryfor determining one or more satellites in view of the wireless device asa function of either the subsequent cached information or initial cachedinformation.
 15. The system of claim 14 further comprising: (g) atransmitter for transmitting assistance data to the requesting entity,the assistance data including information for the one or more satellitesin view or for transmitting an estimated location of a wireless deviceto the requesting entity.
 16. The system of claim 15 wherein therequesting entity is a wireless device or a location based service. 17.The system of claim 14 wherein the circuitry for determining a set ofsatellites further comprises: (i) circuitry for determining at least oneof a location and velocity for each satellite in the set of satellites;and (ii) circuitry for determining an elevation for each satellite inthe set of satellites as a function of the reference location; andwherein each satellite in the set has an elevation greater than apredetermined threshold.
 18. The system of claim 17 wherein thepredetermined threshold is between zero and five degrees above thehorizon of the Earth.
 19. The system of claim 14 wherein thepredetermined time period is approximately five minutes.
 20. The systemof claim 14 wherein the satellites are part of a Global NavigationSatellite System (“GNSS”) selected from the group consisting of: aGlobal Positioning System (“GPS”), Galileo system, GLONASS system,Quasi-Zenith Satellite System (“QZSS”), Beidou satellite system, Compasssatellite system, and combinations thereof.
 21. The system of claim 14wherein the device is selected from the group consisting of: cellulardevice, text messaging device, computer, portable computer, vehiclelocating device, vehicle security device, communication device, andwireless transceiver.
 22. A method for determining one or moresatellites in view of a wireless device comprising: (a) receiving arequest for satellite assistance data from a requesting entity; (b)determining a reference location as a function of the request; (c)determining a first set of satellites in view as a function of thereference location if there is information available from a first cachedset of satellites, the first cached set of satellites being cached forless than a predetermined time period; and (d) determining a second setof satellites in view as a function of the reference location if thefirst cached set of satellites has been cached for greater than thepredetermined time period and determining additional cached informationfor each satellite in the second set of satellites as a function of thereference location.
 23. The method of claim 22 further comprising thestep of: (e) caching information of the second set of satellites in viewfor the reference location.
 24. The method of claim 22 wherein one ormore satellites the first and second sets of satellites are different.25. The method of claim 22 wherein information for cached satellites areselected from the group consisting of: satellite position information,ephemeris information, satellite velocity information, time information,clock correction information, satellite locations, and combinationsthereof.
 26. The method of claim 22 further comprising the steps of: (e)transmitting assistance data to a wireless device, the assistance dataincluding information from the first or second sets of satellites inview; and (f) determining an estimated location of the wireless devicefrom the information.
 27. The method of claim 22 further comprising thesteps of: (e) determining an estimated location of a wireless device asa function of information from the first or second sets of satellites inview; and (f) transmitting the estimated location to the requestingentity.
 28. The method of claim 22 wherein the requesting entity is awireless device or a location based service.
 29. The method of claim 22wherein the predetermined time period is approximately five minutes. 30.The method of claim 22 further comprising the step of cachinginformation for additional sets of satellites at intervals defined bythe predetermined time period.
 31. The method of claim 22 wherein thesatellites are part of a Global Navigation Satellite System (“GNSS”)selected from the group consisting of: a Global Positioning System(“GPS”), Galileo system, GLONASS system, Quasi-Zenith Satellite System(“QZSS”), Beidou satellite system, Compass satellite system, andcombinations thereof.
 32. The method of claim 22 wherein the device isselected from the group consisting of: cellular device, text messagingdevice, computer, portable computer, vehicle locating device, vehiclesecurity device, communication device, and wireless transceiver.
 33. Amethod for determining an estimated location of a wireless devicecomprising: (a) receiving a request for satellite assistance data from awireless device; (b) determining a reference location of the wirelessdevice; (c) determining a set of satellites as a function of thereference location; (d) determining subsequent cached information foreach satellite in the set of satellites if initial cached informationfor each satellite in the set of satellites has been cached for greaterthan a predetermined time period; (e) determining one or more satellitesin view of the wireless device as a function of either the subsequentcached information or initial cached information; (f) transmittingassistance data to the wireless device, the assistance data includinginformation from the one or more satellites in view; and (g) determiningan estimated location of the wireless device from the includedinformation.
 34. A method for determining an estimated location of awireless device comprising: (a) receiving a request for satelliteassistance data from a wireless device; (b) determining a referencelocation of the wireless device; (c) determining a first set ofsatellites in view as a function of the reference location if there isinformation available from a first cached set of satellites, the firstcached set of satellites being cached for less than a predetermined timeperiod; (d) determining a second set of satellites in view as a functionof the reference location if the first cached set of satellites has beencached for greater than the predetermined time period and determiningadditional cached information for each satellite in the second set ofsatellites as a function of the reference location; (e) transmittingassistance data to the wireless device, the assistance data includinginformation from the first or second sets of satellites in view; and (g)determining an estimated location of the wireless device from theincluded information.